The preparations and properties are described of novel anionic and neutral mononuclear biimidazolate (biim), bibenzimidazolate (bibzim), or tetramethylbiimidazolate (tmbiim) manganese(I) and molybdenum(II) complexes of the type [Et 4N][Mn(CO) 2L 2(bibzim)] (L = P(OEt) 3); [Et 4N][Mo(η 5-C 5H 5)(CO) 2-(N) 2] ((N) 2 2− = biim 2−, bibzim 2− tmbiim 2−); [Mn(CO) 4− n L n {H(N) 2}] ( n = 1; H(N) 2 − Hbibzim −; L = P(OMe) 3, PEt 3), ( n = 2; H(N) 2 − = Hbiim −, Hbibzim −; L = P(OPh) 3, P(OMe) 3, P(OEt) 3, P(O iPr) 3; [Mo(η 5-C 5H 5)(CO) 2{H(N) 2}] (H(NN) 2 − = Hbiim −, Hbibzim −, Htmbiim −, in which the heterocyclic anions act as bidentate chelate groups. Treatment of the anionic complexes with MeI gives neutral derivatives of general formula [Mn(CO) 2L 2(Mebibzim)] (L = P(OMe) 3, P(OEt) 3) and [Mo(η 5-C 5H 5)(CO) 2{Me(N) 2}] (Me(N) 2 − = Me-biim −, Mebibzim −, Metmbiim −. Cationic manganese(I) complexes of the type [Mn(CO) 4− n L n {H 2(N) 2}][ClO 4 ( n = 1; H 2 (N) 2 = H 2bibzim; L = P(Ome) 3, PEt 3), ( n = 2; H 2(N) 2 = H 2biim, H 2bibzim; L = P(OPh) 3, P(OMe) 3, P(OEt) 3, P(O iPr) 3) have also been obtained by treating the corresponding neutral complexes with HClO 4. The structures of the complexes have been elucidated by molecular weight determinations, conductance data, and IR spectroscopy.